Sextuple bond

  A sextuple bond is a type of covalent bond involving 12 bonding electrons. The only known molecules with true sextuple bonds (order 6) are diatomic Mo₂ and W₂, in the gaseous phase at very low temperatures. Although diatomic Cr₂ and U₂ have formal structures with twelve-electron bonds, their effective bond orders (derived from quantum chemistry calculations) are less than 5 (quintuple bond). There is strong evidence to believe that no two elements in the periodic table can form a bond with greater order than 6.^[1]

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Mo₂ can be observed in the gas phase at low temperatures (7K) by a laser evaporation technique using molybdenum sheet with for instance near infrared spectroscopy or UV spectroscopy.^[2] Like dichromium, a singlet state is expected from dimolybdenum.^[3] Higher bond order is reflected in shorter bond length of 1.94 Å.

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